The present invention relates to a gripping mechanism for use in robotic application and, more particularly, to an actuation system for actuating an underactuated gripping mechanism in which the number of actuators required is smaller than the number of degrees of freedom of the gripping mechanism.
Many different types and forms of gripping mechanisms are known, providing a variety of functions and uses. Some gripping mechanisms are designed for specific tasks, they are simple, robust, easy to manufacture and lead to simple control schemes. However, they are not flexible and a new gripping mechanism must be designed for each given task. These gripping mechanisms have only a few degrees of freedom and are widely used in industry. Other gripping mechanisms are more flexible and can perform several different tasks. However, they are difficult to manufacture, lead to complex control schemes, include several actuators and can provide only small gripping forces. These gripping mechanisms have several degrees of freedom.
Finally, other gripping mechanisms have an architecture which combines the latter two cases, taking advantage of both through the concept of underactuation. Their design is based on a large number of degrees of freedom but with a reduced number of actuators. Indeed, underactuated gripping mechanisms are defined as those which have fewer actuators than the degree of freedom. This leads to flexible gripping mechanisms without the complexity associated with a large number of actuators.
Underactuation can be achieved using different structural mechanisms. A typical example is described in the Applicants""U.S. Pat. No. 5,762,390, issued on Jun. 9, 1998. A mechanical gripper, described in this patent, has three fingers and three phalanges per finger. The three pivotable phalanges are actuated by one actuator in a flexible and versatile gripping action of three degrees of freedom. The fingers are robust and can provide large gripping forces and perform power grasps and pinch grasps. An additional mechanism is provided to maintain the last phalanx orthogonal to the palm in order to allow the gripper to perform pinch grasps on objects of different sizes. The mechanical gripper including the limited number of actuators permits the fingers to bend independently so that, by actuating some of the actuators and not actuating others, different co-operative bending relationship are achieved.
In addition to the underactuation between the phalanges of a finger, it is also possible to obtain underactuation between the fingers of a gripping mechanism. This will further decrease the number of actuators while maintaining the same number of degree of freedom. This principle has been disclosed for the actuation of many fingers, for example, in U.S. Pat. No. 5,378,033 to Guo et al. and in the literature, see, for example, the article by G. Guo, X. Qian and W. A. Gruver, xe2x80x9cA SINGLE-DOF MULTI-FUNCTION PROSTHETIC HAND MECHANISM WITH AN AUTOMATICALLY VARIABLE SPEED TRANSMISSIONxe2x80x9d, published in the proceeding of the ASME mechanism conference, Phoenix, Vol. DE-45, pp. 149-154, 1992, and the article by M. Rakik entitled xe2x80x9cMULTI-FINGERED ROBOT HAND WITH SELF-ADAPTABILITYxe2x80x9d, published in Robotics and Computer-Integrated Manufacturing, Vol. 5, No. 2-3, pp. 269-276, 1989. In these references, each of the fingers has only one degree of freedom, i.e., the motion of the phalanges is coupled. The combination of the underactuation of the phalanges of a finger and the fingers of a hand is disclosed in the Applicant""s United States Patent. The underactuation between the fingers is performed with the help of a one-input/multi-output differential. The concept of this differential has been introduced in the Applicant""s United States Patent using a lever for two outputs.
It is also possible to orient the fingers with respect to one another (i.e., motion about an axis perpendicular to the palm of the gripping mechanism) with only one actuator by coupling their orientation. This is possible through the use of four-bar mechanisms that connect the base of the fingers. This decreases the number of degrees of the actuation and freedom of the system. This type of coupling has already been suggested in the Applicant""s United States Patent and is provided by gears in U.S. Pat. No. 3,901,547 to Skinner II, and by grooves in the Guo et al. patent.
In order to achieve this underactuation between the fingers in a a differential gripping mechanism, the force of the actuator is to be distributed between the fingers. If a finger grasps an object, the actuator will continue its motion and the other fingers will continue to close with the help of the differential mechanism. Nevertheless, this principle associated with a differential mechanism sometimes limits the performance of the gripping mechanism especially in pinch grasps. It may be desirable, for example, to use only two fingers to perform a pinch grasp and prevent the remainder of the fingers from closing which may potentially disturb the grasp. This is not a problem with a gripping mechanism having multiple actuators because each finger is controllably actuated independently.
Therefore, there exists a need for improved gripping mechanisms which are underactuated between fingers using differential mechanisms and adapted to deactivate predetermined fingers in a closing action when it is desired.
It is also desirable to self-lock the fingers when a gripping mechanism grasps an object. It is especially important when a differential mechanism is used for underactuation between the fingers. An external force acting on one of the fingers may cause displacement not only of the finger receiving the force but also of the remainder of the fingers because all the fingers are associated with the differential mechanism. A lever differential mechanism as described in the prior art is not able to provide the finger self-locking function. Therefore, there exists a need for an actuation system for gripping mechanisms underactuated between fingers, which provides a finger self-locking function.
It is an object of the present invention to provide an actuation system for a gripping mechanism underactuated between a plurality of fingers, using a differential mechanism which is adapted to deactivate predetermined fingers in close/open actions when desired, while actuating the remainder of the fingers in the action.
It is another object of the present invention to provide an actuation system for a gripping mechanism underactuated between a plurality of fingers, using a differential mechanism which is adapted to provide a finger self-locking function in close/open actions.
In one particular embodiment, it is an object of the invention to provide an actuator system for a self-adaptive gripping mechanism with at least ten degrees of freedom which requires two actuations with respect to two co-ordinates that are the force or position of the closing of the fingers and orientation of the fingers, the two co-ordinate being related to improve the performance of the gripping mechanism.
It is a further object of the present invention to provide an actuation system for an underactuated gripping mechanism, which includes an orienting mechanism to rotate the fingers and allows self-locking of the fingers in predetermined orientations and allows, to a certain extent, for more imprecise actuation input for orientation when the fingers are locked in the predetermined orientations.
In general terms, an actuation system for a gripping mechanism underactuated between fingers thereof is provided with a differential mechanism and an orienting mechanism to actuate the fingers in close/open actions and orientation actions respectively, the differential mechanism being adapted to deactivate predetermined fingers in an close/open action when desired.
In more particular terms, an actuation system is provided for a gripping mechanism underactuated between a plurality of fingers at least two of which are rotatable for orientation, each finger having a finger actuation mechanism to actuate the finger in a selective gripping action. The actuation system comprises a differential mechanism operatively connected to the respective finger actuation mechanisms for receiving one power input and transmitting the one power input into a plurality of power outputs to actuate the respective fingers in a close/open action, the differential mechanism including a stop mechanism to deactivate at least a third one of the fingers in the close/open action when desired; an orienting mechanism operatively connected to the rotatable fingers for receiving one rotation input and transmitting the one input into at least two rotation outputs to rotate the respective at least two rotatable fingers in finger orientation.
The number of the plurality of fingers is preferably three, although four or more fingers may be desirable. In the case of four fingers, all fingers may rotate to choose between a radial movement grasp and an opposed finger grasp for cylindrical objects or the like. Preferably, the fingers have at least two or three articulated phalanges and are underactuated.
The stop mechanism is preferably associated with the orienting mechanism so as to stop a close/open action of the third one of the fingers when the two rotatable fingers are oriented to face each other. The differential mechanism is preferably adapted to self-lock the fingers in the close/open action when power for actuating the closing and opening of the fingers is off. The two rotatable fingers are preferably self-locked in predetermined orientations when an orienting motion is not desired.
More especially, in accordance with one embodiment of the invention, an actuation system for actuating a ten-degree of freedom gripping mechanism which includes a palm plane and three underactuated fingers, as described in the Applicant""s U.S. Pat. No. 5,762,390 which is incorporated herein by reference. In addition to the underactuation in the fingers, underactuation between the fingers is performed with the help of a one-input/three-output differential, which comprises two planetary gear trains. The first planetary gear train has a carrier as input and sun gear and an internal gear as outputs. The second planetary gear train has the internal gear of the first planetary gear train as input and a sun gear and an internal gear as outputs. Therefore, the three general outputs are the sun gear of the first planetary gear train, the sun gear of the second planetary gear train and the internal gear of the second planetary gear train. In order to obtain proper distribution of the power, the three outputs should have the same or close to the same output torque. It can be achieved by appropriate ratios of the number of teeth. Three general output gears of the differential mechanism are of equal size and transmit their power to the gears on three screw shafts which are inputs of three transmission screws. The usefulness of the differential is that if one of the fingers is blocked by the object, the other fingers are not blocked and continue to move. Therefore, the finger applies force on the object only when all the fingers have properly made contact with the object.
The three outputs of the differential mechanism are transmitted to the fingers through the transmission screws. Each screw shaft is rotated by the gear on the screw shafts. A linear motion is transmitted to an actuation nut which is threadedly connected to the screw shaft. The rotation of the actuation nut is stopped by a guiding bar, or a driving bar. The driving bar is connected between the actuation nut and the finger actuation mechanism to actuate the closing and opening of the finger.
The closing and opening of the finger is self-locked because the power transmitted from rotation to translation through the transmission screw is not reversible and, therefore, an external force acting on one finger is not able to be transmitted through the differential mechanism to affect the balanced position of the three fingers.
The orientation of two rotatable fingers are rotated with synchronization from one input with the help of a gearing mechanism. A finger gear is attached to each of a rotating basis of the two rotatable fingers. An input gear attached on an orientation shaft directly drives one of the finger gears. A free gear, attached on a free shaft, is driven by the input gear and transmits its motion to the other finger gear. This arrangement allows the respective two finger bases to rotate in opposite directions. Each of the rotatable fingers is able to rotate 90 degrees, from the two fingers facing the third finger which is an orientation fixed finger, to the two fingers facing each other.
The actuation of the gripping mechanism is performed by two actuators. For the opening and closing of the fingers, a first actuator drives the input of the one-input/three-output differential. For the orientation of the fingers, a second actuator drives the orientation shaft.
In one orientation where the two rotatable fingers face each other, the third finger is not used for the grasp and could even potentially disturb the grasp. Therefore, it is stopped in its open position by a mechanism which is added to the orientation shaft. This mechanism comprises a rack attached to the orientation shaft that engages with the output gear of the differential mechanism associated with the orientation-Fixed finger only for the specific configuration in which the two rotatable fingers face each other.
A passive gripping mechanism according to another embodiment of the invention, does not have the two actuators as in the first embodiment, and is driven by a specific external driving apparatus. This apparatus drives the gripping mechanism by a socket that can rotate and advance. The opening/closing of the fingers an the orientation of the fingers are both performed by a socket torque applied by the external driving apparatus. The switching of the power of the socket torque between the two outputs is performed by the socket advance with the help of an indexing mechanism which is part of the passive gripping mechanism. The indexing mechanism works as follows. Each time the socket advance releases and pushes on the indexing mechanism, the travel ends alternate between two different positions. This is possible because of an indexing ring. Depending on the advance of the input shaft, the power is transmitted via a socket to the opening/closing input or to the orientation input. The sockets and male connector are machined for easy alignment.
The orientation of the rotatable fingers is self-locked to predetermine orientations. In order to obtain predetermined self-locked orientations, the orientation shaft is driven via a Geneva mechanism. When the Geneva mechanism is in a moving phase, a pin of a driver is in one of four slots of the Geneva wheel. During this phase, the driver moves the Geneva wheel 90 degrees. When the Geneva mechanism in the dwell phase, the Geneva wheel is locked by a locking disk of the driver. This mechanism allows self-locking of the rotatable fingers, even if they are not driven. It allows for positioning impression of the driver and it also allows free motion of the driver during the dwell phase, which will be proven useful for a switching mechanism.
In a third embodiment of the invention, the gripping mechanism is actuated by fluid power which may be either hydraulic or pneumatic. The fingers are the same is in the other embodiments. The underactuation between the fingers is performed by a fluid power system instead of the gearing system. The fluid power is partitioned in three outputs, which emulates the one-input/three-output differential. Each of these outputs powers one of the three piston cylinders that drive the three fingers. The self-locking feature of the transmission screws is replaced by controllable check valves which ensure that the fingers will not go back unless the power to open the fingers is activated. To orient the fingers, a rotational fluid actuator activates the orientation shaft. As an alternative to a mechanical blocking mechanism, a solenoid valve may be connected to one piston cylinder which is associated with the orientation fixed-finger to stop the fluid supply to the piston cylinder to deactivate the opening and closing of the finger. The solenoid valve may be controlled by a switch connected to the orientation shaft so that the solenoid valve is activated to shut off the fluid supply only when the two rotatable fingers are oriented to face each other.
The actuation system according to the present invention provides a practical approach to establish a link between the differential mechanism for actuating the closing and opening of the fingers and the orienting mechanism for rotating the fingers so that the performance of the gripping mechanism underactuated between the fingers thereof is significantly improved, especially in pinch grasps. The advantages of the actuation system also includes the self-locking of each finger when the power is off, which is important to the gripping mechanism underactuated between the fingers. Other features and advantages will be more apparent with reference to the details of the preferred embodiments to be described below.
According to another aspect of the invention, there is provided a gripping mechanism comprising at least three fingers, at least two of which are rotatably counted for orientation on a palm, an actuation mechanism causing each of the finger to open and close, a differential mechanism operatively connected to the respective finger actuation mechanisms for receiving a grasp power input and transmitting the one power input into a plurality of power outputs to actuate the respective fingers in a close/open action, an orienting mechanism operatively connected to the respective rotatable fingers for receiving one orientation input and transmitting the one input into at least two rotation outputs to rotate the respective at least two rotatable fingers in a finger orientation, a two-degree of freedom power input having two degrees of freedom for receiving mechanical actuation power external to the gripping mechanism, and a switching mechanism selectively connecting the two-degree of freedom power input to the grasp power input of the differential mechanism or to the orientation input of the orienting mechanism in response to movement of the two-degree of freedom power input in one of the two degrees of freedom.
Preferably, the two-degree of freedom power input comprises rotation and translation, the power grasp input being derived from the rotation. The two-degree of freedom power input may be a single power shaft input, and the switching mechanism comprises an axially displaceable connector mounted to the power shaft input for rotating therewith, an indexing mechanism connected to the power shaft input and axially movable sequentially between a neutral position, a grasp power input position, a neutral position and an orientation position, wherein the axially displaceable connector engages the grasp power input of the differential mechanism or the orientation input of the orienting mechanism in response to movement of the axial translation of the two-degree of freedom power input. While the fingers have preferably to least three degrees of freedom and the actuation mechanism differentially drives each degree of freedom of the finger, it is possible to provide fewer or more degrees of freedom to the fingers.